The phase behavior of amphiphilic copolymer-alkanol-water ternary systems was investigated for triblock copolymers of similar molecular weight and the same hydrophilic block [poly(ethylene oxide), E] but having different hydrophobic blocks [poly(propylene oxide), P, or poly(n-butylene oxide) = poly(tetrahydrofuran), T]. The alkanol used (butan-1-ol) was comparable in terms of chemical composition to the hydrophobic segments. A rich phase behavior was obtained for the polymer with the P middle block (Pluronic F127, E100P70E100) : five different one-phase regions, i.e., micellar (L-1) and reverse micellar (L-2) solutions, and (micellar) cubic (I-1), hexagonal (H-1), and lamellar (L-alpha) lyotropic liquid crystalline regions, were detected. The microstructure in the liquid crystalline regions was established from small-angle X-ray measurements; I-1 was found to be primitive cubic. The alkanol molecules are most likely anchored with their OH- group at the E-P interface, increasing the apparent volume of the P blocks relative to that of the E blocks, and thus causing a decrease in the polymer layer curvature from spherical to cylindrical. Only a single one-phase region, extending from the water to the alkanol corner, was observed in the E100T27E100 ternary phase diagram. The extent of the one-phase regions decreased for both E100T27E100 and E100P70E100 systems when the alkanol molecular weight increased.